New thermal decomposition pathway for TATB

Understanding the thermal decomposition behavior of TATB (1,3,5-triamino-2,4,6-trinitrobenzene) is a major focus in energetic materials research because of safety issues. Previous research and modelling efforts have suggested benzo-monofurazan condensation producing H2O is the initiating decomposition step. However, early evolving CO2 (m/z 44) along with H2O (m/z 18) evolution have been observed by mass spectrometric monitoring of head-space gases in both constant heating rate and isothermal decomposition studies. The source of the CO2 has not been explained, until now. With the recent successful synthesis of 13C6-TATB (13C incorporated into the benzene ring), the same experiments have been used to show the source of the CO2 is the early breakdown of the TATB ring, not adventitious C from impurities and/or adsorbed CO2. A shift in mass m/z 44 (CO2) to m/z 45 is observed throughout the decomposition process indicating the isotopically labeled 13C ring breakdown occurs at the onset of thermal decomposition along with furazan formation. Partially labeled (N18O2)3-TATB confirms at least some of the oxygen comes from the nitro-groups. This finding has a significant bearing on decomposition computational models for prediction of energy release and deflagration to detonation transitions, with respect to conditions which currently do not recognize this oxidation step.

TATB is an important explosive compound because of extensive use in munitions.Typically formulated with a small percentage of polymer to modify properties, the material has been utilized world-wide in weapon systems.TATB is widely viewed as one of the most stable insensitive high explosives (IHE)s, as it is not easily detonated by external stimuli 1 .It does not undergo the thermal sequence of deflagration-to-detonation (DDT).It requires a proper detonation chain to initiate, so handling the material is relatively free from accidental initiation if proper safety methods are followed.One aspect of this safety envelope is how the material responds to temperature extremes; whether this material becomes more sensitive and is no longer safe to handle when subjected to abnormal thermal environments.This issue has been the subject of extensive research interest for close to 50 years 2,3 .The objective has been to understand the behavior experimentally to construct computer models predicting behavior for any thermal exposure condition [4][5][6][7][8] .
The consensus in the literature shows TATB decomposes thermally through the furazan reaction network shown in Fig. 1 [9][10][11][12] .Several studies have indicated the first step in the decomposition is condensation of adjacent amino-and nitro-groups forming H 2 O and the furazan ring.Spectroscopic evidence shows this sequence probably proceeds stepwise until all substituents are condensed.Subsequently, the rings fall apart leading to light-gas formation, such as C 2 N 2 and HNCO, HCN, etc [10][11][12][13][14][15][16][17][18] .The molecular profile of TATB as it relates to DDT transitions has traditionally been overlooked in the literature and our study highlights novel molecular pathways that occur as IHE is heated above its stability limit.These pathways can help constrain the physiochemical properties of current and future IHE compounds and allow prediction of the behavior and safe handling of energetic materials.This study has established a new understanding of IHE decomposition as it pertains to DDT transitions and lays the foundations for linking complex molecular processes to kinetic and thermodynamic measurements of IHE.

Results
Figure 2 shows the thermal decomposition of TATB as measured by SDT-MS.The upper part of the figure shows the heat flow and weight loss as a function of temperature.The heat flow exhibits the major exotherm with a maximum at 347.1 °C (1 °C/min heating rate) with a smaller exotherm as a leading-edge shoulder.The weight loss profile shows most of the decomposition occurs during heat flow for these two exotherms.Using constant heating rate thermal decomposition methods, we have detected early evolving H 2 O, but in addition, early evolving CO 2 as well.The bottom part of Fig. 2 shows the mass spectra for various light gases evolved during the decomposition.All the gases align with the sharp major exotherm, with some also aligning with the leading-edge shoulder.The blue trace shows early evolving H 2 O, consistent with the furazan decomposition route.However, the red trace shows early evolving CO 2 which is not consistent with the furazan condensation and indicates an oxidation process is occurring.The carbon source of this oxidation is either adventitious carbon from absorbed species (impurities or CO 2 ) or from the ring of the TATB.To understand this source, 13 C 6 -TATB was synthesized and was further examined using pyrolysis GC-MS in evolved gas analysis (EGA) mode EGA-GC-MS.
Figure 3 shows the ion behavior with respect to time from the EGA-GC-MS analyses of the TATB and 13 C 6 -TATB heated isothermally at 330 °C.The EGA profiles show the total ion chromatogram (TIC) for the unlabeled TATB and 13 C 6 -TATB along with extracted ion chromatograms (EIC) for m/z 44 and 45.In both cases, there is an early intense pulse of gases occurring in the first 30 s of heating arising from minor impurities and instrument background.The bulk of gases evolving from TATB thermal decomposition start ~ 5-10 min later.For TATB, early evolving CO 2 , m/z 44, is evident; for 13 C 6 -TATB, there is very little CO 2 , m/z 44, produced.The EICs for 45 m/z represent 13 CO 2 forming from oxidation of the TATB phenyl ring.For unlabeled-TATB there is almost no intensity for m/z 45 while for the 13 C 6 -TATB, the profile shows m/z 45 ion evolving both early and late.The residual 44 m/z CO 2 in the 13 C 6 -TATB profile is due to 94 % isotopic purity 19 .
It is significant that the 13 C material decomposes about twice as fast as 12 C material.The faster decomposition was verified using thermal analysis (simultaneous mass loss and heat flow) at both isothermal and constant heating rate conditions using methods described elsewhere 20 .What drives the increased decomposition rate  in the 13 C 6 -TATB remains unknown, however it may be related more to changes in the TATB crystal structure and alignment of carbon and nitro groups to promote oxidation than an inverse kinetic isotope effect due to the minor mass defect between 12 C and 13 C.For heating rates of 0.5 to 10 °C/min, decomposition rate of 13 C 6 -TATB maximizes about 6 °C lower 20 .
An attempt to completely substitute 18 O into the nitro-group on the TATB only was partially successful yielding less-than-complete 18 O-substituted TATB.However, the substitution was successful enough to use analytically and demonstrate at least a partial source of the oxidation of the carbon.Figure 4A shows the EGA mass spectra from 5 to 55 min of the crude (N 18 O 2 ) 3 -TATB also isothermally heated at 330 °C.For CO 2 , m/z 44 is the predominant feature in the low mass range.The ion m/z 46 is also evident from CO 18 O, which would occur with the only partially labeled material, indicating the early evolving CO 2 is likely occurring due to involvement in the oxidation by nitro group on TATB.The 18 O labeling mostly produced one 18 O atom on the NO 2 group of TATB.www.nature.com/scientificreports/ The peak at m/z 46 could also arise from NO 2 , however the EGA mass spectra from unlabeled TATB produces a minor peak at m/z 46.This is likely due to the high reactivity of NO gas, that is consumed in oxidation reactions.The incorporation of only one 18 O into the CO 2 molecule indicates it is likely a direct oxidation by NO 2 , either as a gas or neighboring molecule, transferring two O atoms to the C of the phenyl ring.Low intensity m/z 48 was observed in the (N 18 O 2 ) 3 -TATB and likely formed from the small amount of double labeled NO 2 present in the sample further supporting the strong oxidant as the driving force.The addition of 18 O into the other major thermal decomposition products was observed as well.For example, as appears in Fig. 4B, TATB and the furazans all increased by 2 amu.

Discussion
The formation of CO 2 from the generation of NO 2 was verified with the 13 C and 18 O labeled TATB experiments and support the auto-oxidation of TATB with heating.The extracted ion chromatograms from the EGA experiments with isothermal heating reveal rapid oxidation takes place when TATB is heated to decomposition temperatures.This occurs early in the isothermal experiments at 330 °C and the release of NO 2 and CO 2 gas coincide as a portion of the TATB is oxidized.However, the intensity in the mass spectrometer from NO 2 gas is minor.This is likely due to instrument response and possibly because it is instantaneously consumed by redox reactions as TATB decomposes to CO 2 and N 2 gas.The nitro groups on TATB are predicted to be the weakest bond in the TATB structure 21,22 and NO 2 scission into the gas-phase is predicted at temperatures > 330 °C.This process occurs rapidly in the first min of isothermal heating at 330 °C.The formation of isocyanic acid (HNCO) would be favored in this initial oxidation event if NO 2 gas is oxidizing the carbon ring and attaching oxygen to a carbon next to an amine group 22,23 .Nitrogen oxide (NO) gas is also an oxidant that may be playing a role in the auto-oxidation process, and Fig. 2 shows the verification of co-evolution by SDT-MS.In related 15 N isotopically labeled experiments we were able to tentatively identify N 2 as a breakdown product 16 .Other studies have verified the presence of N 2 gas during thermal decomposition of TATB 13 .Figure 5 shows the auto-oxidation reaction.The NO 2 in the graphic is meant to represent the start of the oxidation step, but not necessarily the active oxidant, as discussed above.
Much effort has been put into trying to understand how solid-phase-only reactions initiate.For TATB, kinetics and modelling studies suggest the first step is due to H 2 O formation, (probably through the well-established poly nitro decomposition process with gamma position hydrogens 24,25 ) which would implicate the furazan reaction network shown Fig. 1.This has been consistent with analytical isolation of reaction products [9][10][11][12] .However, these new results show an oxidation reaction is occurring early and the source of the carbon is the TATB ring itself.This reaction has never been shown before for TATB, but NO 2 involvement in oxidation at the onset of explosive decomposition (often leading to detonation) has often been implicated with nitro-only type explosives 26,27 .In the TATB case, the two reaction mechanisms, furazan formation and oxidation by NO 2 appear to be in competition early on as a cascade of redox reactions occur.The formal oxidation numbers for the nitro and amino groups on the TATB are + 3 and − 3, respectively.If a furazan molecule/s/ forms, the oxidation number for both N atoms would have to shift to + 1.A series of oxidation/reduction reactions would have to occur for this to happen and represent an overlooked step in the thermal decomposition of TATB.The steps leading to this early oxidation, CO 2 generation, dehydration and furazan formation remain unclear, and the generation of NO 2 gas in addition to the inter/intra molecular electron shuttling could play a role.The TATB molecules are firmly locked into a planar array with other TATB molecules through interactions such as hydrogen bonding.These arrays are stacked in a tertiary structure, also stabilized by hydrogen bonding, attributed to the remarkable insensitivity of the TATB to external stimuli and hence the label as an IHE 28,29 .How the solid-state structure can reorganize to allow for this oxidation to occur is a matter for future examinations.
These results could also be critical for understanding why TATB (and formulations) do not go from deflagration to detonation (DDT), another feature desirable about IHE.Most monomolecular military type explosives have nitro groups as the functional substitution to produce explosive power.The amino group was introduced to provide better thermal stability 30,31 .That introduction could be the key to keeping TATB from DDT.Our research www.nature.com/scientificreports/reveals this newly elucidated oxidation mechanism, which is proposed to drive explosives to detonations 26,27 , in the case of TATB, has competition from the furazan condensation reaction for available NO 2 .This is enough competition early on to keep the burning reaction from accelerating to create the wave front needed for detonation.Other, nitro-based only explosives do not have this competition for the oxidation source.These results lay the foundation for understanding auto-oxidation thermal decomposition in nitro amine based IHEs and a step towards the mechanistic understanding of what makes a robust IHE.

STD measurements
Constant heating rate: TA Instruments Q600 STD Simultaneous Differential Scanning Calorimetry-Thermogravimetric Analysis coupled with a Pfeiffer Vacuum Thermostar GSD 320

Synthesis
The 13 C label was introduced by converting 13 C labeled aniline to 13 C trichloro benzene followed by wet amination synthesis to make 13 C 6 -TATB ( 13 C 6 H 6 N 6 O 6 ), characterized by SS-NMR, FTIR, MS, DSC; 94% isotopically pure, 92 wt% chemically pure.The 18 O labeled was introduced by the same synthesis method as in delineated elsewhere 19 with modifications of trichloro benzene nitrated with KN 18 O 3 .Only partially substituted products were isolated due to exchange with the acids during synthesis-1 18 O-nitro group: 32.44%; 2 18 O-nitro groups: 7.87%; 3 18 O-nitro groups: 1.04%.

Py-GC-MS
Pyrolysis GC-MS (Py-GC-MS) experiments were performed using an Agilent 7890B GC coupled to a 7010B MS/MS mass spectrometer and a Frontier Lab pyrolyzer (EGA/PY-3030D), auto-shot sampler (AS-2020E) and selective sampler (SS-2010E); a steel column (3 m × 150 µm inside diameter) was used to measure evolving gases with no chromatographic separation; silver foil was used generate pseudo-confined samples, comparable to the SDT-MS experiments with 50-µm pinholes; scan on the first quadrupole mass spectrometer from mass 29 to 650, using a 600 ms dwell time.Samples were heated isothermally at 330 °C for 100 min 32 .

Figure 5 .
Figure 5. Phenyl ring carbon oxidation decomposition pathway for TATB driven by direct intra-molecular reactions or via inter-molecular reactions with NO 2 from adjacent TATB molecules.